How do astronomers know the shape of our galaxy (the Milky Way), even though it is not possible to take a photograph of it because to do that we would have to go away from it?

The clues we have to the shape of the Milky Way are:

1) When you look toward the Galactic Center with your eye, you see a long, thin strip. This suggests a disk seen edge-on, rather than a ellipsoid or another shape. We can also detect the bulge at the center. Since we see spiral galaxies which are disks with central bulges, this is a bit of a tipoff.

2) When we measure velocities of stars and gas in our galaxy, we see an overall rotational motion that differs from random motions. This is another characteristic of a spiral galaxy.

3) The gas fraction, color, and dust content of our Galaxy are like other spiral galaxies.

So, overall, it's a pretty convincing argument. Of course, we have to assume our galaxy is not completely unlike the other galaxies we see—once a civilization has accepted that it does not occupy any special place in the Universe, arguments about similarity seem sensible.

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